The known spring hinges as characterized above cannot be made economically in large quantities (by modern manufacturing methods) without finishing operations, if an exact guidance is to be ensured at the hinge axis and if the overall height of the spring hinge is to be minimized. In this connection, it is necessary to meet the requirements of the hingemaker as regards manufacturing technology, and of the eyeglass maker as regards processing technology, and the requirements of the optician and the wearer of the eyeglasses regarding the function of the hinge. Safety is also important for the wearer.
The requirements of the eyeglass maker regarding manufacturing technology result from the various technologies used in making spectacle frames. For instance, plastic frames are either milled from acetate slabs, which are mostly extruded, or are injection-molded from propionate granules, or vacuum-cast from epoxy resins. A spring hinge which can be used in all these different manufacturing processes has not yet been provided. Such spring hinge should also be usable with a metal bow.
In all manufacturing processes, a good joint between the spring hinge and the metal core of the plastic bow or the metal bow is a requirement for a satisfactory and economical processing.
In plastic bows made from slab material, the metal cores of the bows and the spring hinge are shot into the bow material, which has been thermally plasticized. A dense joint between the spring housing and the bow core is required, as well as a neat pressing of the partly bulged plastic material. In the thermoplastic processing of the spring hinge, the spring housing must be tightly joined under pressure to the bow core and must provide a seal against the injection mold. In addition, the housing mold must permit a satisfactory removal of the injection-molded part from the mold. In the vacuum-casting of casting resin, the absolute pressure and vacuum tightness of the seal between the housing and the bow core is of great significance. In case of a leak, low-viscosity casting resin might penetrate in the evacuated casting mold into the interior of the housing, so that the spring hinge would not be operative.